I can agree with that, but then, the patients I see over 50 are usually on so many different medications just to keep them alive that blaming the longer life span on body weight is a tad simplistic. Medical science has progressed to the point that we can keep people alive much longer than they would have experienced even 50 years ago.
Blood thinners, BP medications, insulin, thyroid hormone, and anti-depressants are the usual barrel of pill boxes they bring into the clinic.
I am not sure that qualifies as "healthy in old age".
Also, there are very few people on the planet who even lift weights seriously past the age of 30 which means larger individuals are usually represented as OBESE and not very muscular. That is a huge difference.